Grundy Newbrain repaired
Grundy Newbrain repaired
Defect:
- Dead.
Analysis:
- The acid of the capacitors has corroded some pcb tracks and pitches of the main pcb.
- One of the two flat cable (main pcb -> keyboard/vfd pcb) previously repaired cause short circuits and doesn’t work.
- Removing the Filter capacitor “RIFA” in the external PSU (removal made 7 months ago)
Repair # 1:
- Replaced all the electrolytic capacitors.
- Rebuild the PCB tracks corroded by the acid of the capacitors.
- Replaced the flat between the main PCB and the keyboard/vfd pcb.
The computer now turns on but the VFD (Vacuum Fluorescent Display) displays weird characters and the composite video signal is dead.
Repair # 2:
I have used a Z80 NOP generator that i have made some years ago for repairing ZX Spectrum/Sinclair and that causes the Z80 to process only “NOP” instructions
and consequently on the address lines we can see with a oscilloscope a perfect square wave.
With this simple “trick” i have immediately found the faulty component that break one of the address lines of the CPU (A1). The exact same fault was found by Thomas Gutmeier.
- Replaced 1 x 74LS257 (IC 412)
The computer now works perfectly.
Gallery of the repair:
Download: Grundy NewBrain Schematics (1532)
Categories: Projects / Repairs, Today, Various
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